Means for indicating the logarithmic value of a magnitude



Dec. 8, 1953 P. B. VANDERLYN MEANS FOR INDICATING THE LOGARITHMIC VALUE OF A MAGNITUDE Filed Jan. 19, 1951 F/GZ lnvnfiar PHILIP BAR/VET VA NDERLYN Patented Dec. 8, 1953 MEANS FOR INDICATING THE LOGARITH- MIC VALUE OF A MAGNI'IUDE Philip Barnet Vanderlyn, Ealing, London, England, assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application January 19, 1951, Serial N 0. 206,792

Claims priority, application Great Britain January 25, 1950 3 Claims.

This invention relates to means for indicating the logarithmic value of a magnitude and it relates particularly but not exclusively to means for indicating the output of a radio receiver or the like on a logarithmic scale, for example, for the purpose of programme monitoring.

The object of the invention is to provide improved means for indicating the logarithmic value of a magnitude.

According to the invention, means are provided for indicating the logarithmic value of magnitude comprising means for generating a periodic exponentially varying voltage, voltage comparison means, means for feeding to said comparison means said. exponentially varying voltage and a voltage proportional to said mag-- nitude, pulse generating means coupled to said comparison means to generate pulses of a ration commencing with equality between said voltages, and means for feeding said pulses to indicating means to indicate the duration of said pulses.

In order that the said invention may be clear- 1y understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings in which:

Figure 1 is a diagram of a circuit embodying the invention, and

Figure 2 is a diagram illustrating the operation of the circuit of Figure 1.

Referring to Figure 1, the circuit shown may be employed, for example, in a radio programme monitor, for indicating the peak level of trans mitted sound on a logarithmic scale. The circuit includes an electron discharge tube of the double triode type, comprising triodes i and 2, the cathodes of said triodes being coupled by a capacitor 3 and the anode of the triode i being connected to the control electrode of the triode 2 via a circuit comprising a capacitor i and shunt resistor 5. are connected to the positive H. T. terminal shown via resistors l and 8 respectively. Bias for the triode I is obtained from a potentiometer comprising resistors 9 and It, the junction of which is connected to the control electrode of tube I. Bias for the control electrode of tube 2 is derived from the potentiometer formed by resistors l and and a resistor l2 connected between ground and resistor 5. tube is connected to ground via resistors l3 and M in series and the cathode of tube 2 is connected to ground via a resistor [5. The elements so far described constitute a cathode coupled multivibrator which is arranged to generate The anodes of the triodes l and 2 The cathode of short positive voltage pulses, of for example about 50 volts amplitude at approximately one millisecond intervals. The pulses generated by the multivibrator are applied via a blocking capacitor It to the control electrode of an electron. discharge tube I8, and a circuit is provided which establishes the positive peaks of. the pulses at about volts positive with respect to the lower end of the cathode resistor IQ of tube l8. Said circuit includes a diode 2i! the anode of which is connected to the control electrode of tube 18, and the cathode of which is biased by being connected to a suitable point on a potentiometer comprising resistors 2!, 22 and 23 connected in series between the positive H. T. terminal and ground. A resistor 24 is connected between the anode of diode 2t and the junction of resistors 22 and 23. The'anode of tube I8 is connected directly to the positive H. T. terminal. A capacitor 25 is connected in shunt with resistor l9. The output of tube It is taken from across capacitor 25 and is fed to the con trol electrode of a triode 21 forming part of a double triode electron discharge tube, the other triode of which is indicated by reference. nu-

meral 28. The cathodes of triodes 2? and 28 are both connected to the anode of an electron discharge tube 29, the cathode of which is connected to ground via a resistor so and the control electrode of which is biased by a voltage derived from a point on the potentiometer formed by resistors 3i and 32 connected in series between ground and the positive H. T. terminal shown. The circuit includes a further double triode electron discharge tube comprising a pair of triodes 33 and 3:2. The anode of triode 27 is connected to the control electrode of triode 33 and the anode of triode 28 is connected directly to the positive H T. terminal. A potentiometer formed by resistors 35 and Bi serves to provide suitable bias for the control electrode of triode 33. The cathodes :oftriodes 33 and 34 are both connected to groundvia resistors 38 and 39 in series. The anode of triode 33 is connected via a resistor it! and a current meter 4! to the positive H. T. terminal, and the anode of triode at is connected directl to the positive H. T. terminal. The anode of triode 3,3 is also coupled to the control electrode of triodefi i by means of resistor 22, the said control electrode being biased by voltage derived froma potentiometer formed by resistors it and con.- nected in series between the positive i-I. liter.- minal and ground. Smoothing capacitors 4t, 4?,

3 48 and 49 serve to reduce voltage variations at points where such variations are undesirable.

The operation is as follows:

Tube 288 is non-conducting "at rall times except when the short positive pulses from the anode of tube 2 are applied to its control electrode. When the said pulses are applied and the tube H8 becomes conducting, the voltage. generatedacross cathode resistor I9 produces'acharge-nn napacitor 25, this charge being established relatively rapidly. When the tube |8-hecomes:non conduoting following the cessation ot each pulse the charge leaks away via the "cathode resistor Ill and the voltage at the cathode .end of-resistor .|.9

falls exponentially. Accordingly a periodic exponentially varying voltage is @applied ito tthe control electrode of triode 21. A signal voltage proportional to the magnitude to be indicated, for example the peak level of transmitted sound,

is applied between terminals 50 and 5|, so that the voltage of the control electrode "of tube Z8 "with respect 'to terminal 5| is proportional "to said magnitude. It is 'arranged that the maximumwalue'of' the voltage-of the 'control'electrod'e oftriode'ZB is less than the maximum voltage "of the exponential -waveform of the "voltage applied tothe ccntrol'electrode'oftriodefl'. The sum :of :the 'currents through triodes '21 and 2-8 is "substantially constant "and equal to the "anode "current "of tube '29, the said'anode'current "being Jdetermined :by the potential "of the control electrode o'f'tiib'e29 and 'thevalue'of the cathode resistor '30. "Iriodes "21 and f 28 save to "compare the exponential voltage with the signal voltage or the control electrode or triode 18,1. e. with the aboveementioned magnitude. During the initial, part of each cycle *of' the exponential volt age the voltage of thecontrolelectrod'e of "triode I! .iishigher than that of the control 'electro'de .dftriode18 and accordinglytheanodefcurrent or tube 29 passes through .triode 21. 'TheVoltage of the control electrode 'of "triode '21 then "falls in an exponential manner until it "becomes "less than the voltage for the :control electrode of .triode l8, and thereupon the anode current-of tube '29 passes through "triode 28 instead 'of through itrio'de '21. Accordingly, the "voltage at the anode of the trickle *2! rises, for example from "190 -volts "to 220 -volts. The time required TOTthis change "inyolta'geof triode *2! "is equal to the time taken for the "exponential voltage to 'fallthrough'the range of voltagesreqiiiredto pause thecurrent to *i'low'wholly through triode-'28 instead "of throughtridde 21. Triodes 33 andm form =a multivibrator in which ino 'curre'nt flows throughthe triode 33 when its:control electrode is at the initialvoltage 70f the anode .of rtriode '27, 301' :example 2190 ivolts, rbut "when the said amide voltagerrises itorahout 1210 :volts tthe multi- =vibrator switches lover and tall current .flows through triode 33, the .instant at which the multivibrator switches over being that at which .the -.exponential voltage .is -.substantially equal .tothe signal voltage of .the-controlelectrodepf tube 28. Themultivibrator comprising'the tubes 33.and13'4 has thus'two stable conditions in which the tubes are alternately conducting. During the itime that triode '33 is conducting, current 'flows throughthe meterfll, andaecordingly currentllows 'inmeter *4 I "so' long as theexponential voltage is less 'thanthe 'said's'ignal'voltage between terminals 5|! and 5 I. "Upon each "application of afpulse to "thecontrol electrode-oftube I78 the exponential voltage rises rapidly and the circuit is restoredto its 'original "condition. "and current ceases to flow in meter 4| until the exponential voltage again falls to the value of the voltage between terminals 50 and 5|. The curirent in'meterMtthus .eon'sistsmfipulseslhaving a constant amplitude determined by the resistances 38 and 39 and lasting from the time at which the exponential voltage is equal to the signal voltageof .tube 28 until the completion of .apy'cle-of operation of the multivibrator triodes I and 2. The frequency of the pulses is such tthat the imeterireading does not vary at each pulse hut is proportional to the mean current flowing "through the meter. The time taken for the circuit to.be.r.estored to its original condition 'by each pulse from the multivibrator I,

(ZiiSEShOl'trfCOlHDWCi-Zd with the time of a complete yrlleof operations and the efiect on the current inthemeter 4| can be neglected to a first approximation.

Referring -now to Figure 2, wherein voltage is plotted against time, E "represents the signal voltage between terminals 150 and 51, "and V1: represents the exponentially varying voltage applied to the control electrode of triode '21. Va represents the initial value of the exponential voltage attimeto and V1 re'pr'eserits'th'e value of the exponential voltage at the instant when a pulse is about to be applied to tube .|8 ta-to representing the time of a complete cycle ofthe exponential waveform. 'The'sh'aded area' i're'presents the constant current flowing through the meter H between the times t1'a'n'd t2, t1 being the time at which the exponential voltage Vais equal to the voltage E. It can'b'e shownitliat themeancurrent im'through triode'33 an'd'th'erefore the-mean current'through meter 1|, is giv'en by:

that is to say the meter reading is proportional to the logarithm .of the signal =voltage of triode .28, said voltage beingproportional to thamagnitude concerned.

.In :the above equation CR .is the time .constant of :the capacitor 25andresistor [9 and .the logarithmic range of voltage over which-Ithe circuit operates is dependent on .the ratio Fonpurposes .of calibration therefore the repetition .period tzto is arranged -.to be variable and for :this purpose resistor 44 is adjustable as shown. An adjusta'bletapping-on resistor A8 its connected to thepositi-ve H. T. terminal via ?a resistor 52. .Adjustment-cf the said tapping-en- .ables the -.voltage to which the exponential waveform is asymptotic to -.be varied for the -.purpose of removing the effect of .contactpotential am .the rectifier circuit from whichvoltageEis derived. Thacurrent inmeter-4| 'atmaximum.in- .dicationds l determined by adjusting the value of resistor 39.

There will beaa small constant-current at all times through .meter 4| -via:resistors 40, and 45. This current does 7 not .-afiect the .operation of the circuit, butif desired can easily'be avoided by means well -knowninthe art.

'I'hecircuit illustrated is suitableionuse with a single TH. T. .supply and may convenientlybe employed in portable equipment.

A circuit'acc'or'ding to'theinventionmay have application to computing apparatus. .For example, division "and multiplication may be efcuits to an electrical subtracting or adding circuit as the case may be.

I claim:

1. Means for indicating the logarithmic value of a magnitude, comprising means for generating a periodic exponentially varying voltage, voltage comparison means, means for feeding to said comparison means said exponentially varying voltage and a voltage proportional to said magnitude, pulse generating means cou pled to said comparison means to generate pulses of a duration commencing with equality between said voltages, indicating means, and means for feeding said pulses to said indicating means to indicate the duration of said pulses.

2. Means for indicating the logarithmic value of a magnitude, comprising means for generating a periodic exponentially varying voltage, a pair of electric discharge tubes each having a control electrode, a common cathode circuit for said tubes, means for applying said periodic voltage to the control electrode of one of said tubes, means for applying a voltage proportional to said magnitude to the control electrode of said. other tube, means to maintain one of said tubes conducting and the other nonconducting, means for changing the conducting conditions of said tubes when said voltages applied to said tubes are equal, means for deriving an output voltage from one of said tubes, and means for producing from said output voltage, pulses of duration determined by the change in the conducting conditions of said tubes, indicating means, and means for feeding said pulses to said indicating means to indicate the duration of said pulses.

3. Means according to claim 2, said means for producing said pulses comprising a multivibrator having a pair of electron discharge tubes each provided with a control electrode, means for maintaining said multivibrator in two stable conditions with one of the tubes thereof conducting and the other tube thereof nonconducting, means for applying said output voltage to the control electrode of one of the tubes of said multivibrator, and means including said indicating means in the anode circuit of one of the tubes of said multivibrator.

PHILIP BARNET VANDERLYN.

References Cited in the file of this patent UNITED STATES PATENTS Number 

